Transformer saturation and unusual system oscillation in capacitively coupled half-bridge or full-bridge forward converters: causes, analyses, and cures

In capacitively coupled bridge converters, duty-ratio unbalanced can cause transformer saturation and resulting transistor destruction. Independently, modulation of the transistor storage time by variations of collector current can cause low-frequency system oscillation which leads also to transformer saturation, and additionally causes low-frequency conducted EMI which is difficult to filter. A resistor shunting the coupling capacitor is suggested as a simple but effective cure for both problems. Mathematical analyses for both phenomena, a design procedure for selecting the proper value of shunt resistance, and experimental results that confirm the analyses are presented.